FIELD OF THE INVENTION
The invention relates to a ceramic-coated product, in particular a ceramic coated component, for use in a hot gas duct, especially in industrial gas turbines. The invention furthermore relates to a process for producing a product having a thermal barrier layer.
A product of that type has a base body of a metal alloy based on nickel, cobalt or iron. Products of that type are primarily used as a component of a gas turbine, in particular as gas turbine blades or heat shields. The components are exposed to a hot gaseous flow of aggressive combustion gases. They must therefore be capable of withstanding very heavy thermal stresses. It is furthermore necessary for those components to be resistant to oxidation and corrosion.
Primarily for moving components, e.g. gas turbine blades, but also for static components, there are also mechanical requirements. The power and the efficiency of a gas turbine in which components that can be subjected to hot gas are used, rise with increasing operating temperature. In order to achieve high efficiency and high power, those parts of the gas turbines which are especially subjected to the high temperatures are coated with a ceramic material. The latter acts as a thermal barrier layer between the hot gas flow and the metallic substrate.
The metallic base body is protected from the aggressive hot gas flow by coatings. That being the case, modern components usually have a plurality of coatings, each of which fulfills specific requirements. A multilayer system is thus involved.
Since the power and efficiency of gas turbines rise with increasing operating temperature, efforts are constantly being made to achieve higher gas turbine performance by improving the coating system.
A first approach with a view to this improvement is in optimizing the adhesion layer. U.S. Pat. No. 4,321,310 discloses the application of an MCrAlY adhesion layer in such a way that it has a low degree of surface roughness. A layer of aluminum oxide is then formed thereon in order to achieve thereby a substantial improvement in the adhesion of the thermal barrier layer.
U.S. Pat. No. 4,880,614 discloses incorporation of a high-purity aluminum layer between the MCrAlY adhesion layer and the metallic base body. That aluminum is used to form a dense Al.sub.2 O.sub.3 layer on the adhesion layer in order to increase the life of the coated component.
U.S. Pat. No. 5,238,752 discloses an adhesion layer of nickel aluminides or platinum aluminides. A layer of aluminum oxide is formed on that adhesion layer. The thermal barrier layer is applied thereon.
U.S. Pat. No. 5,262,245 discloses that the aluminum oxide layer is formed as an oxidation layer from the material of the base body. For that purpose, the base body has a nickel-based alloy which has strongly oxide-forming alloy constituents.
U.S. Pat. No. 4,676,994 discloses the application of a layer that forms aluminum oxide to a base body. Aluminum oxide is formed on the surface of this layer. A dense ceramic layer is applied thereon by evaporation coating. This ceramic layer is formed of a dense substoichiometric ceramic material. It may be an oxide, nitride, carbide, boride, silicide or a different refractory ceramic material. A thermal barrier layer is applied to that ceramic layer.
The great majority of the above U.S. patents indicate that the thermal barrier layer has a columnar microstructure in which the crystallite columns of the columnar microstructure extend perpendicular to the surface of the base body. Stabilized zirconium oxide is indicated as the ceramic material. Suitable stabilizers include calcium oxide, magnesium oxide, cerium oxide and, preferably, yttrium oxide. The stabilizer is needed in order to prevent a phase transition from the cubic to the tetragonal and then monoclinic crystal structure. In essence, the tetragonal phase is stabilized to about 90%.
In U.S. Pat. No. 4,321,311, voluminous defects are provided in the thermal barrier layer in order to reduce stresses which are produced in the thermal barrier layer when the temperature changes, as a result of the fact that the base body and the thermal barrier layer have different coefficients of thermal expansion. The thermal barrier layer has a columnar structure with gaps between the individual columns of the coating of zirconium oxide stabilized with yttrium oxide.
Another proposal for solving the problem of stress when confronted with temperature variation is indicated in U.S. Pat. No. 5,236,787. There, an intermediate layer of a metal/ceramic mixture is interposed between the base body and the thermal barrier, in which the metallic proportion of this intermediate layer increases in the direction of the base body and to decrease in the direction of the thermal barrier layer. Conversely, the ceramic proportion should be low close to the base body and high close to the thermal barrier layer. The thermal barrier layer proposed is a zirconium oxide stabilized with yttrium oxide and having some proportion of cerium oxide. The thermal barrier layers are deposited on the base body by plasma spraying or PVD methods. The proportion of the yttrium oxide stabilizer is from 8 to 20% by weight.
U.S. Pat. No. 4,764,341 discloses the bonding of a thin metal layer to a ceramic. Nickel, cobalt, copper and alloys of these metals are used for the metal layer. In order to bond the metal layer to the ceramic substrate, an intermediate oxide such as aluminum oxide, chromium oxide, titanium oxide or zirconium oxide is applied to the ceramic substrate. At a sufficiently high temperature, this intermediate oxide forms a ternary oxide through oxidation by incorporating an element from the metallic coating.